Features: send Play/Stop/Next/Previous command to MPD, decrease/increase
volume using the mouse wheel, show the current volume, status and title as
a tooltip when passing the mouse over the plugin, show a simple playlist
window upon middle-click, permitting to select a track to play, toggles
repeat/random features in the right-click menu, launch configurable client
(gmpc, xterm -e ncmpc,..) through right-click menu.
